Employee Retirement Benefit Plans - Additional Information (Detail) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Transatlantic Holdings Incorporated
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligation	$ 79.0	$ 65.5
Fair value of plan assets	56.9	49.1
WWSC Holdings, LLC
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligation		10.0
Fair value of plan assets		11.7
Parent Company
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligation	44.1	32.3
Fair value of plan assets	3.5	$ 3.3
Executive Retirement Plans
Defined Benefit Plan Disclosure [Line Items]
Expected future employer contributions, next fiscal year	57.0
Estimated other comprehensive income attributable to defined benefit retirement plans, before tax, that will be reclassified to expenses in 2020	21.0
Other comprehensive losses attributable to retirement plans, before tax	$ 15.4